The present invention relates generally to electrical fuses having means for indicating a fused condition, and relates particularly to current-limiting power fuses with such indicating means.
Current-limiting power fuses generally have a cylindrical housing of insulating material which is closed at each end by a metal terminal cap. A main fusible element, usually a silver ribbon, extends inside the housing between the terminals. The space around the fuse element is filled with silica sand. When current through the fuse element exceeds the rating of the fuse for a sufficient time, the element melts or fuses at one or more points, with the resulting formation of arcs there. The arcs progressively melt the sand and form channels of fulgurite in it. The relatively high resistance fulgurite eventually suppresses any significant flow of current through the fuse.
Because the melting of current-limiting fuses of the type described above is silent and does not normally result in an obvious change in the outward appearance of the fuse, it is common to provide on the fuse an indicating means by which it can be readily determined visually that the fuse has melted, or fused. Such indicating means typically include an insulating support cup mounted to the inside face of one of the terminal caps over an opening in the cap. A cup-shaped, insulating indicator button is situated in a telescoping fashion inside the support cup. A coil spring inside the button away from the bottom of the support cup. The button is held in place against the force of the spring by a fusible restraining wire which passes into the button through a small insulating bushing in the bottom of the support cup, under an attachment tab protruding from the inside top of the button, and out again through the bushing in the bottom of the support cup. The wire is fixed under tension by a tapered pin pressed into the passage in the bushing. The loose wire length extending from the bushing is electrically connected to the immediate terminal cap, while the other length extends through the silica sand in the housing to connection at the other terminal cap.
When the main element of the fuse melts, the line voltage appears across the terminal caps. This results in melting of the restraining wire inside the button. The unrestrained button is now pushed out through the opening in the terminal cap by the spring. The protruding portion of the button provides a ready visual indication that the fuse has melted.
Current limiting fuses are sometimes located where they may be exposed to an ambient combustible gas. The melting of the indicator wire under such conditions could cause ignition of the ambient gas by the arcing inside the indicator button and result in an explosion.